1. Field of the Invention
The present invention relates to an optical modeling method, and more particularly to an optical modeling method in which a three-dimensional model is created by exposing a photo-curable resin to a light beam.
2. Description of the Related Art
Recently, with the spread of three-dimensional computer-aided design (CAD) systems, an optical modeling system has come into general use in which a three-dimensional form is created in a virtual spacing on a computer, a photo-curable resin is exposed with a light beam on the basis of CAD data of the three-dimensional object, and is modeled to an actual three-dimensional model by using the three-dimensional CAD system. The optical modeling system comprises the steps of creating a plurality of cross sectional data by slicing CAD data at regular spacings; hardening the photo-curable resin into layers by scanning the surface of a liquid type photo-curable resin with the irradiation of laser light on the basis of each cross sectional data as shown in FIG. 17A; and modeling a three-dimensional model by layering a photo-curable resin layer sequentially. As an optical modeling method, a free liquid surfacing method is widely known in which a liquid type photo-curable resin is reserved in an open top reservoir beforehand, and a photo-curable resin layer is layered while gradually sinking a modeling table disposed near the liquid surface of the photo-curable resin from a free liquid surface of the photo-curable resin.
In this optical modeling method, polymerization shrinkage is produced by the curing of a photo-curable resin, and curing shrinkage is produced by heat distortion in which resin, which is once heated to a high temperature by polymerization heat during the curing of the photo-curable resin, is then cooled to room temperature. This shrinkage due to resin curing causes a problem in that an object to be optically modeled is distorted, thereby deteriorating modeling accuracy.
Especially when a region incorporating therein a plurality of pixels is exposed at one time (surface exposure) and cured into a plate shape, as shown in FIG. 17B, an object to be optically-modeled is distorted and warped to form a downward concave shape relative to a direction in which photo-curable resin layers are layered.
As a method of inhibiting such distortions caused into an object to be modeled, Japanese Patent Application Laid-Open (JP-A) No. 6-114949 discloses a photo-curable modeling method in which a direction that the photo-curable resin is scanned with a light beam is inverted every one other layer or every several other layers of the resin to be scanned by the light beam. In the photo-curable modeling method, it is noted that the photo-curable resin layer easily curves with a portion of the layer to be scanned with the light beam at the end of the scanning, put inside the layer, directions in which the photo-curable layers to be layered are scanned with the light beam are inverted, and stress that attempts to flex in one direction is offset, whereby distortion can be inhibited. However, in the photo-curable modeling method, since each photo-curable resin layer is formed by scanning and exposing the resin with a single light beam, a problem is caused in that much time is taken for modeling.
Further, Japanese Patent Application Laid-Open (JP-A) No. 5-154924 discloses a method of modeling using a layered plate (hereinafter, “a layered plate modeling method”) in the photo-curable modeling method, in which a layer having an uncured portion (i.e., unexposed portion) left thereon is formed, the uncured portion underlying an upper layer is cured at the same time when the upper layer is cured, and the layered plate is formed. In the layered plate modeling method, shrinkage stress caused by the curing of the upper layer and shrinkage stress caused by the curing of the uncured portion are offset to thereby control distortion of the object to be optically modeled. However, in the layered plate modeling method, since the uncured portion of the layer is gradually cured, a problem is caused in that configuration of the object to be optically modeled changes with times, thereby deteriorating modeling accuracy.